Developments in the optical fiber communications field have been rapid. However, the technology still is undergoing major shifts in direction. For example, earlier generation fiber systems were designed to operate at wavelengths of about 0.8 .mu.m, and current systems operate at 1.3 .mu.m. Now there is growing interest in systems having an operating wavelength of about 1.55 .mu.m to take advanatage of the loss window that exists in silica-based optical fiber in that wavelength region. Another example of a major shift which is driven by demand for higher bandwidths is that from multimode to single mode fibers.
Although desired for their large bandwidth capabilities and small size, light-transmitting optical fibers are mechanically fragile, exhibiting low-strain fracture under tensile loading and degraded light transmission when bent. The degradation in transmission which results from bending is known as microbending loss. As a result, cable structures have been developed to protect mechanically the optical fibers.
A cable for use in a duct must be capable of withstanding tensile loads applied when the cable is pulled into the duct and stresses caused by bends. Cable structures which have been developed for optical fibers include loose tube, stranded and ribbon cables. For a description of loose tube cables, see, for example, D. Lawrence and P. Bark "Recent Developments in Mini-Unit Cable" published at pp. 301-307 of the Proceedings of the 32nd International Wire and Cable Symposium, 1983. See also U.S. Pat. No. 4,153,332.
Ribbon cable comprises one or more ribbons with each including a plurality of optical fibers disposed generally in a planar array. In U.S. Pat. No. 4,078,853 which issued to R. Kempf et al on Mar. 14, 1978 is shown a cable which includes a core of ribbons surrounded by a loose-fitting plastic inner tubular jacket. A plastic outer jacket is reinforced with strength members which are encapsulated in the outer jacket to achieve tight coupling therewith.
In some situations, especially duct systems which include many bends such as those in loop plant in urban areas, greater tensile loads are expected. An improved optical communications cable which is suitable for such use is disclosed in U.S. Pat. No. 4,241,979 which issued on Dec. 30, 1980 in the names of P. F. Gagen and M. R. Santana. A bedding layer, about which strength members are wrapped helically, is added between plastic extruded inner and outer jackets to control the extent to which the strength members are encapsulated by the outer jacket. The cable includes two separate layers of strength members, which are wrapped helically in opposite directions. Under a sustained tensile load, these two layers of strength members produce equal but oppositely directed torques about the cable to insure the absence of twisting.
Ribbon cable has a number of attractive features. One is the realative ease of array connectorization. Array connectors shown, for example, in U.S. Pat. No. 3,864,018 can be factory installed and can save much time over that required for single fiber joining techniques. A further advantage is that a higher fiber density can be achieved per unit of cable cross-section than in a stranded cable.
In another type of optical communications cable, a plurality of optical fibers are enclosed in an extruded plastic tube to form a unit and a plurality of these tubed units are enclosed in a common extruded plastic tube which is enclosed in a sheath system. Generally, the optical fibers which are enclosed in each unit tube are stranded together about a central strength member. A central strength member is used because it is relatively easy to assemble into the cable. Also, the cable is more easily bent if it has a central strength member rather than strength members which are incorporated into the sheath system. However, when such a cable is bent, the central strength member may in some instances compress one or more of the fibers against the tube and cause damage thereto.
Generally, optical fiber cables of the prior art, such as ribbon and stranded and loose tube, suffer from the disadvantage of having the ribbons, the stranded units or the tubes manufactured on a separate line. In stranded cable, for example, a plurality of units which priorly have been enclosed individually in tubes and stranded are fed into a line which applies the common tube and the outer jacket. Each of the units must be made separately on another line and inventoried until a plurality of them can be associated together in the common tube. Because the ribbon or tubed core is generally stranded with a predetermined lay, its manufacture and the assembly of the ribbons or tubes into the core involves the use of relatively heavy rotating apparatus which is undesirable from a manufacturing standpoint.
Further complicating the cable situation is the introduction of a waterblocking filling material into the cable core in order to prevent the incursion of water. A viscoelastic waterblocking material which has been used in the past is disclosed in U.S. Pat. No. 4,176,240 issued on Nov. 27, 1979, in the name of R. Sabia. Typically, the waterblocking materials in use do not yield under strains experienced when the cable is made or handled. This prevents the movement of the optical fibers within the cable and the fibers buckle because they contact, with a relative small periodicity, a surface of the unyielding filling material. The smaller the periodicity of the fibers in contacting such an unyielding surface, the greater the microbending loss. This is overcome somewhat by stranding the cable which allows the fibers under stress to form new helices to avoid microbending losses. A grease-like filling composition having a relatively low critical yield stress is disclosed in application Ser. No. 697,054 filed on Jan. 31, 1985, in the names of C. H. Gartside III et al (now U.S. Pat. No. 4,701,016).
Clearly, what is needed is a cable for optical fiber transmission which departs from those used in the past. That cable should be one which can be made inexpensively relative to present costs and which is relatively compact. Also, the cable structure should be one which inhibits the introduction of undue stresses which would lead to microbending losses in the optical fibers. It is believed that the prior art does not include such a cable for which there is a long felt need in order to provide low cost optical fiber communications.